US 7171983 B2
Improved microfluidic devices, systems, and methods allow selective transportation of fluids within microfluidic channels of a microfluidic network by applying, controlling, and varying pressures at a plurality of reservoirs. Modeling the microfluidic network as a series of nodes connected together by channel segments and determining the flow resistance characteristics of the channel segments may allow calculation of fluid flows through the channel segments resulting from a given pressure configuration at the reservoirs. To effect a desired flow within a particular channel or series of channels, reservoir pressures may be identified using the network model. Viscometers or other flow sensors may measure flow characteristics within the channels, and the measured flow characteristics can be used to calculate pressures to generate a desired flow. Multi-reservoir pressure modulator and pressure controller systems can optionally be used in conjunction with electrokinetic or other fluid transport mechanisms.
1. A method of controlling the movement of fluid through an intersection between a first channel and a second channel in a microfluidic device, the method comprising:
applying a first pressure differential across the first channel with a first independently variable pressure modulator, and
applying a second pressure differential across the second channel with a second independently variable pressure modulator.
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This application is a continuation of U.S. patent application Ser. No. 09/792,435, filed Feb. 23, 2001, now U.S. Pat. No. 6,915,679 B2, which claims the benefit under 35 USC 119(e) to U.S. Provisional Patent Application Nos. 60/184,390, filed Feb. 23, 2000, and 60/216,793, filed Jul. 7, 2000, which are hereby incorporated herein by reference in their entirety for all purposes.
The present invention is generally related to analytical tools for the biological and chemical sciences, and in particular, provides microfluidic devices, systems, and methods for selectively transporting fluids within microfluidic channels of a microfluidic network, often using a plurality of selectively variable pressures.
Microfluidic systems are now in use for the acquisition of chemical and biological information. These microfluidic systems are often fabricated using techniques commonly associated with the semiconductor electronics industry, such as photolithography, wet chemical etching, and the like. As used herein, “microfluidic” means a system or device having channels and chambers which are at the micron or submicron scale, e.g., having at least one cross-sectional dimension in a range from about 0.1 μm to about 500 μm.
Applications for microfluidic systems are myriad. Microfluidic systems have been proposed for capillary electrophoresis, liquid chromatography, flow injection analysis, and chemical reaction and synthesis. Microfluidic systems also have wide ranging applications in rapidly assaying compounds for their effects on various chemical, and preferably, biochemical systems. These interactions include the full range of catabolic and anabolic reactions which occur in living systems, including enzymatic, binding, signaling, and other reactions.
A variety of methods have been described to effect the transport of fluids between a pair of reservoirs within a microfluidic system or device. Incorporation of mechanical micro pumps and valves within a microfluidic device has been described to move the fluids within a microfluidic channel. The use of acoustic energy to move fluid samples within a device by the effects of acoustic streaming has been proposed, along with the use of external pumps to directly force liquids through microfluidic channels.
The capabilities and use of microfluidic systems advanced significantly with the advent of electrokinetics: the use of electrical fields (and the resulting electrokinetic forces) to move fluid materials through the channels of a microfluidic system. Electrokinetic forces have the advantages of direct control, fast response, and simplicity, and allow fluid materials to be selectively moved through a complex network of channels so as to provide a wide variety of chemical and biochemical analyses. An exemplary electrokinetic system providing variable control of electro-osmotic and/or electrophoretic forces within a fluid-containing structure is described in U.S. Pat. No. 5,965,001, the full disclosure of which is incorporated herein by reference.
Despite the above-described advancements in the field of microfluidics, as with all successes, still further improvements are desirable. For example, while electrokinetic material transport systems provide many benefits in the micro-scale movement, mixing, and aliquoting of fluids, the application of electrical fields can have detrimental effects in some instances. In the case of charged reagents, electrical fields can cause electrophoretic biasing of material volumes, e.g., highly charged materials moving to the front or back of a fluid volume. Where transporting cellular material is desired, elevated electrical fields can, in some cases, result in a perforation or electroporation of the cells, which may effect their ultimate use in the system.
To mitigate the difficulties of electrokinetic systems, simplified transport systems for time domain multiplexing of reagents has been described in WO 00/45172 (assigned to the assignee of the present invention), the full disclosure of which is incorporated herein by reference. In this exemplary time domain multiplexing system, structural characteristics of channels carrying reagents can, at least in part, regulate the timing and amount of reagent additions to reactions (rather than relying solely on the specific times at which pumps are turned on and/or valves are actuated to regulate when and how much of a particular reagent is added to a reaction). While other solutions to the disadvantageous aspects of electrokinetic material transport within a microfluidic system have been described, still further alternative fluid transport mechanisms and control methodologies would be advantageous to enhance the flexibility and capabilities of known microfluidic systems.
Regardless of the mechanism used to effect movement of fluid and other materials within a microfluidic channel network, accuracy and repeatability of specific flows can be problematic. There may be variations in, for example, electroosmotic flow between two chips having similar designs, and even between different operations run on a single chip at different times. Quality control can be more challenging in light of this variability, as accurate control over microfluidic flows in applications such as high throughput screening would benefit significantly from stable and reliable assays.
In light of the above, it would be advantageous to provide improved microfluidic devices, systems, and methods for selectively transporting fluids within one or more microfluidic channels of a microfluidic network. It would be desirable if these improved transport techniques provided selective fluid movement capabilities similar to those of electrokinetic microfluidic systems, while mitigating the disadvantageous aspects of the application of electrical fields to chemical and biochemical fluids in at least some of the microfluidic channels of the network.
It would also be beneficial to provide improved devices, systems, methods and kits for enhancing the accuracy, reliability, and stability of microfluidic flows within a microfluidic network. It would be beneficial if these enhanced flow control techniques provided real-time and/or quality control feedback on the actual flows, ideally without relying on significantly increased system complexity or cost.
The present invention generally provides improved microfluidic devices, systems, and methods. The devices and systems of the invention generally allow flexible and selective transportation of fluids within microfluidic channels of a microfluidic network by applying, controlling, and varying pressures at a plurality of reservoirs or ports. By modeling the microfluidic network as a series of nodes (including the reservoirs, channel intersections, and the like) connected together by channel segments, and by determining the flow resistance characteristics of the channel segments, the fluid flows through the channel segments resulting from a given pressure configuration at the reservoirs can be determined. Reservoir pressures to effect a desired flow profile may also be calculated using the network model. A simple multi-reservoir pressure modulator and pressure controller system can optionally be used in conjunction with electrokinetic or other fluid transport mechanisms. The invention also provides techniques to avoid fluid mixture degradation within a microfluidic channel by maintaining sufficient oscillation to avoid separation of the fluid mixture when no gross movement of the fluid is desired. Microfluidic systems and methods having viscometers or other flow sensors are particularly useful for determining pressures so as to hydrodynamically induce a desire to flow in response to a measured flow within a microfluidic channel. Regardless of the mechanism used to effect movement of fluids within a microfluidic network, the techniques of the present invention may be used to provide feedback on the actual flow and/or network system characteristics, allowing (for example) more accurate, stable and reliable assays.
In a first aspect, the invention provides a microfluidic system comprising a body defining a microfluidic channel network and a plurality of reservoirs in fluid communication with the network. The network includes a channel. A plurality of pressure modulators are also included, each pressure modulator providing a selectably variable pressure. A plurality of pressure transmission lumens transmit the pressures from the pressure modulators to the reservoirs so as to induce a desired flow within the channel.
Generally, the lumens will transmit the pressures to the ports with significantly less resistance to the lumen flow than the resistance of the channel to the associated microfluidic flow. Each pressure modulator will typically be in fluid communication with an associated port via an associated lumen. In many embodiments, a network flow controller will be coupled to the pressure modulators and will send signals to the pressure modulators so that the modulators vary the pressures. The network controller will generally include channel network data which correlates the channel flows with the pressures from the pressure modulators.
In some embodiments, the network will comprise a plurality of microfluidic channels in fluid communication at channel intersections. The intersections and reservoirs will define nodes coupled by channel segments. The network data can indicate correlations between the flows in the channel segments and the plurality of pressures.
In other embodiments, a network data generator may be coupled to the network controller. The network data generator may comprise a network flow model, a viscometer coupled to the channel, and/or a network tester adapted to measure at least one parameter indicating the pressure-flow correlation. The pressure controller or controllers will often make use of signals from pressure sensors so as to provide a pressure feedback path. Optionally, the pressure controllers may include calibration data correlating drive signals with the resulting reservoir pressures. Preferably, the pressure modulators will comprise pneumatic displacement pumps.
Typically, at least one sample test liquid will be disposed in the channel network. A pressure-transmission fluid can be disposed in the lumens, with a fluid/fluid-pressure-transmission interface disposed therebetween. Typically, the pressure-transmission fluid will comprise a compressible gas, which can compliantly couple the pressure modulators with the channel flow.
Typically, the system will include at least four independently variable pressure modulators. Preferably, the system will make use of at least eight independently variable pressure modulators. A pressure interface manifold can be used to releasably engage the microfluidic body, the manifold providing sealed fluid communication between the lumens and the associated reservoirs. Ideally, a plurality of electrodes will also be coupled to the microfluidic network with an electrokinetic controller coupled to the electrodes so as to induce electrokinetic movement of fluids within the network. In general, when a hydrodynamic pressure differential is used to move fluid within the microfluidic network, the pressure differential will be significantly greater than a capillary pressure of fluids within the reservoirs.
In another aspect, the invention provides a body defining a microfluidic channel network with a plurality of reservoirs in fluid communication with the network. The network includes a first channel. A plurality of pressure modulators is also provided, with each pressure modulator in fluid communication with a reservoir for varying a pressure applied thereto. A network flow controller is coupled to the pressure modulators. The network controller comprises channel network data correlating a flow within the first channel and the pressures from the pressure modulators. The network controller independently varies the pressures from the pressure modulators in response to a desired flow within the first channel in the network data.
Optionally, the system may further include means for generating the network data coupled to the network controller. The network data generating means may comprise a model of the network, a viscometer, an electrical resistance sensor for sensing electrical resistance within the network, or the like.
In another aspect, the invention provides a microfluidic system comprising a body defining a microfluidic channel network and a plurality of ports in fluidic communication with the network. The network includes a first channel. A network flow controller generates independent desired pressure signals in response to a desired flow within the first channel. A plurality of pressure modulators coupled to the network flow controller are each in fluid communication with an associated reservoir. A pressure controller with calibration data couples the pressure modulators with the network controllers. The pressure controllers transmit drive signals to the pressure modulators in response to desired pressure signals from the network flow controller and the calibration data.
In a first method aspect, the invention provides a microfluidic method comprising transmitting a first plurality of pressures to an associated plurality of reservoirs using a plurality of pressure transmission systems. A first flow is induced within a first microfluidic channel of a microfluidic network in response to the first pressures. A second plurality of pressures in determined so as to effect a desired second flow within the first microfluidic channel. The determined second plurality of pressures are applied with the pressure transmission systems and the second flow is induced within the first microfluidic channel with the second pressures.
The methods of the present invention are particularly well suited for precisely combining selected fluids within a microfluidic network, such as for multiport dilution in which concentrations of first and second fluids from first and second reservoirs can be combined at different concentrations.
In another method aspect, the invention provides a microfluidic method comprising determining pressure-induced flow characteristics of a microfluidic channel within a microfluidic network. A first plurality of pressures are derived from the characteristics of the microfluidic network so as to provide a first desired flow in a first microfluidic channel. The first desired flow is induced by applying the first pressures to a plurality of ports in communication with the microfluidic network.
In yet another method aspect, the invention provides a method for use with a fluid mixture which can degrade when held stationary. The method comprises introducing the fluid mixture into a microfluidic channel of a microfluidic network. The mixture is maintained by oscillating the fluid mixture within the channel. The maintained fluid mixture is then transported along the channel.
While analysis of the microfluidic network based on the known channel geometry can significantly facilitate calculation of pressures to be applied for generation of a desired hydrodynamic flow, work in connection with the present invention has shown that the complex nature of the flows within a microfluidic channel can make calculation of effective fluid viscosity within a microfluidic network highly problematic. Specifically, the flows within a single channel of a microfluidic network may include differing dilutions of test fluids separated by a plurality of different buffering solutions, and the like. To over come this complication, the invention often makes use of viscometers and other flow sensing systems to determine actual flow characteristics from a known microfluidic driving force. Based on these measurements, a desired flow may then be generated hydrodynamically by adjusting the appropriate reservoir pressures.
In a related method aspect, the invention provides a microfluidic method comprising inducing flow within a microfluidic channel of a microfluidic network. The flow is measured and a pressure is calculated from the measured flow so as to generate a desired flow. The desired flow is generated within the channel by applying the calculated pressure to the microfluidic network.
The flow is optionally measured by generating a detectable signal within the flow at a first location, and by measuring a time for the signal to reach a second location. The signal may comprise a change in a fluid of the flow, particularly where the first location comprises an intersection between a plurality of microfluidic channels. Such a change in the flow may be initiated hydrodynamically by applying a pressure pulse to a reservoir in communication with the intersection, and/or electrokinetically by varying an electrical field across the first intersection. Optionally, a plurality of detectable signals from a plurality of channel intersections may be sensed as each of these signals reaches the second location. In many embodiments, a signal will comprise a change in an optical quality of fluid in the flow. For example, the signal may comprise a change in a concentration of a dye from a channel intersection, as described above. Alternatively, where the fluid comprises a photobleachable dye, the dye may be photobleached by a laser at the first location with the photobleaching sensed at the second location. Many of these methods will allow a speed of the flow to be determined, particularly when a distance between the first and second locations is known. In some embodiments, the speed of the flow may be determined by, for example, Dopler velocimetry, tracer particle videography, or the like. Ideally, a viscosity of the flow can be calculated using a first pressure (which induces the measured flow) and the speed of the flow. This viscosity can then be used in determination of the calculated pressure so as to generated the desired flow.
In a related system aspect, the invention provides a microfluidic system comprising a body defining a microfluidic channel network and a plurality of reservoirs in fluid communication with the network. The network includes a microfluidic channel. A viscometer is coupled to the channel for determining a viscosity of a flow therein.
In yet another system aspect, the invention provides a microfluidic system comprising a body defining a microfluidic channel network and a plurality of reservoirs in fluid communication with the network. The network includes a microfluidic channel. A plurality of pressure modulators are in fluid communication with the reservoirs. A sensor is coupled to the channel for transmission of flow signals in response to flow within the channel. The controller couples the sensor to the pressure modulators. The controller transmits pressure commands in response to the flow signals to provide a desired flow.
In yet another aspect, the invention provides a microfluidic system comprising a body defining a microfluidic channel network and a plurality of reservoirs in fluid communication with the network. The system also includes means for selectively and independently varying pressures within the reservoirs. The pressure varying means is in fluid communication with the reservoirs.
In yet another aspect, the invention provides a microfluidic method comprising inducing a perturbation in a flow through a microfluidic channel of a microfluidic network by applying a pressure transient to the microfluidic network. A characteristic of the flow or microfluidic network is determined by monitoring progress of the perturbation.
The pressure transient may conveniently be applied by spontaneous injection of an introduced fluid into an injection channel of the microfluidic network. Such spontaneous injection may draw the introduced fluid into the injection channel using capillary forces between the injection channel and the introduced fluid.
Typically, the perturbation will comprise a change in a material of the flow downstream of an intersection. This change will often comprise a change in quantity of a fluid from a first channel, with the pressure transient being applied at the first channel.
The use of pressure induced flow perturbations may be used to determine flow or network characteristics in systems having flow that is pressure induced, electrically induced, or any mixture of flow inducing mechanisms. Typically, flow characteristics such as effective flow viscosity, flow speed, and the like may be determined. In some embodiments, network characteristics such as flow resistance of one or more channels may be determined.
The progress of the perturbation may be monitored at least in part with a sensor disposed downstream of a perturbation source location (such an intersection of channels). A speed of the flow may be determined from, for example, a time interval extending from the pressure transient to detection of the perturbation at the sensor location, and from a distance along the channel or channels extending from the source location to the sensor location. More complex analyses are also possible, such as determining a second speed of a second flow. This second speed may be generated in response to a time interval defined in part by detection of a second flow perturbation, and a second distance defined in part by a second perturbation source location (such as a second channel intersection). As the different speeds along intersecting channels may be determined, the amount of materials combined from different channels at an intersection may be calculated.
In a related system aspect, the invention provides a microfluidic system comprising a body having channel walls defining a microfluidic network. A pressure transient generator is in communication with a channel intersection of the microfluidic network for initiation of a flow perturbation. A sensor is coupled to the flow within the network at a sensor location. A processor coupled to the pressure generator and the sensor determines a characteristic of the flow or the network in response to detection of the perturbation at the sensor location.
FIGS. 8 and 8A–8D graphically illustrate an enzyme assay using a multi-reservoir pressure controlled microfluidic system, and more specifically:
The present invention generally makes use of a multi-reservoir pressure controller coupled to a plurality of independently variable pressure modulators to effect movement of fluids within microfluidic networks. By selectively controlling and changing the pressure applied to the reservoirs of a microfluidic device, hydrodynamic flow at very low flow rates may be accurately controlled within intersecting microfluidic channels. Such pressure-induced flows can help to decrease (or entirely avoid) any detrimental effects of the electrical fields associated with electrokinetic transportation methods, such as sample bias, cell perforation, electroporation, and the like. Additionally, such pressure-induced microfluidic flows may, through proper chip design, reduce flow variabilities as compared to electrokinetic techniques through the use of pressure differentials (and/or channel resistances that are significantly greater than flow variations induced by secondary effects, such as inflow/outflow capillary force differentials within the reservoirs). Advantageously, the pressure-induced flows of the present invention may also be combined with electrokinetic and/or other fluid transportation mechanisms thereby providing composite pressure/electrokinetic microfluidic systems.
The techniques of the present invention will often make use of data regarding the network of channels within a microfluidic device. This network data may be calculated using a model of the microfluidic network, measured by testing a microfluidic device, sensed using a sensor, and/or the like. The network data will often be in the form of hydrostatic resistances along microfluidic channel segments connecting nodes, with the nodes often being intersections between channels, ports or reservoirs, connections between channel segments having differing cross-sectional dimensions and/or flow characteristics, and the like. As used herein, the term “reservoir” encompasses ports for interfacing with a microfluidic network within a microfluidic body, including ports which do not have cross-sections that are much larger than the microfluidic channel to enhance fluid capacity.
By selectively controlling the pressure at most or all of the reservoirs of a microfluidic system, very small flow rates may be induced through selected channel segments. Such small pressure-induced flows can be accurately controlled at flow rates which might be difficult and/or impossible to control using alternative fluid transportation mechanisms. Advantageously, the present invention may provide flow rates of less than 0.1 nanoliters per second, the flow rates often being less than 1 nanoliters per second, and the pressure induced flow rates typically being less than 10 nanoliters per second within the microfluidic channel.
To accurately apply the pressures within the microfluidic network, the invention generally makes use of a pressure transmission system having relatively large lumens coupling the pressure modulators to the reservoirs of the microfluidic device, with the pressure transmission lumens ideally containing a compressible gas. Pressure is often transmitted through this relatively low resistance pressure transmission system to fluids disposed within the reservoirs of the microfluidic system via a gas/fluid interface within the reservoir. The resistance of the microfluidic channels to the fluid flows therein is typically much greater than the resistance of the pressure transmission lumens to the associated flow of compressible gas. Generally, the channel resistance is at least 10 times the transmission system resistance, preferably being at least 100 times, and ideally being at least 1000 times the transmission system resistance of the compressible gas used to induce the channel flows. In other words, a response time constant of the pressure transmission system will generally be lower than the time constant of the channel network, preferably being much lower, and ideally being at least one, two, or three orders of magnitude lower. The head space of a fluid (for example, in the pressure modulator pump and/or in the port or reservoir) times the resistance of the fluid flow (for example, in the channels or lumens) may generally define the response time constant.
Surprisingly, it is often advantageous to enhance the resistance of the microfluidic channels to provide the desired relative resistance factors. The channels may have reduced cross-sectional dimensions, pressure drop members (such as a small cross-section pressure orifice, a flow restricting substance or coating, or the like), and/or lengths of some, most, or even all of the microfluidic channel segments may be increased by including serpentine segment paths. As the resistance of the pressure transmission system can be several orders of magnitude less than the resistance of the channels, pressure differentials can be accurately transmitted from the pressure modulators to the reservoirs of the microfluidic device. Additionally, reduced transmission system resistances can help to enhance the response of the pressure system, providing a faster response time constant.
Referring now to
Modulator bank 14 generally provides independently selectable pressures to the lumens of tubing 20 under the direction of a controller(s) 22. Feedback may be provided to controller 22 from pressure sensors 24, as will be described hereinbelow. Processor 22 will often comprise a machine-readable code embodied by a tangible media 26, with the machine-readable code comprising program instructions and/or data for effecting the methods of the present invention. Processor 22 may comprise a personal computer having at least an Intel Pentium® or Pentium II® processor having a speed of at least 200 MHz, 300 MHz, or more. Tangible media 26 may comprise one or more floppy disks, compact disks, or “CDs,” magnetic recording tape, a read-only memory, a random access memory, or the like. In some embodiments, the programming instructions may be input into controller 22 via a disk drive or other input/output system such as an internet, intranet, modem reservoir, or the like. Suitable programs may be written in a variety of programming languages, including the LabView™ language, as available from National Instruments of Austin, Tex. Controller 22 transmits drive signals to modulator bank 14, ideally via an RS232/RS485 serial connection.
In addition to tubing 20, pressure transmission system 16 includes a manifold 28. Manifold 28 releasably seals the lumen of each tube 20 with an associated reservoir 18 of microfluidic device 12. Tubing 20 may comprise a relatively high-strength polymer such as polyetheretherketone (PEEK), or a polytetrafluoroethylene (such as a Teflon™ material), or the like. The tubing typically has an inner diameter in a range from about 0.01″ to about 0.05″, with a length from about 1 m to about 3 m. A “T” connector couples the pressure output from each pressure modulator to an associated pressure sensor 24.
Each modulator 14 a, 14 b . . . generally comprises a pump or other pressure source which pressurizes the compressible gas within the lumen of associated tubing 20. The modulators preferably comprise positive displacement pumps, with the exemplary modulators comprising a piston which is selectively positioned within a surrounding cylinder by an actuator. Preferably, the actuators are adapted to allow accurate positioning of the piston in response to drive signals from controller 22, the exemplary actuators comprising stepper motors. The exemplary piston/cylinder arrangement is similar to a syringe. Exemplary modulator banks may be provided by (or modified from components available through) a variety of commercial sources, including Kloehn of Las Vegas, Nev., Cavaro of Sunnyvale, Calif., and the like.
Microfluidic device 12 is seen more clearly in
The microfluidic devices or systems of the present invention typically include at least one microscale channel, usually at least two intersecting microscale channel segments, and often, three or more intersecting channel segments disposed within a single body structure. Channel intersections may exist in a number of formats, including cross intersections, “T” intersections, or any number of other structures whereby two channels are in fluid communication.
The body structures of the devices which integrate various microfluidic channels, chambers or other elements may be fabricated from a number of individual parts, which when connected form the integrated microfluidic devices described herein. For example, the body structure can be fabricated from a number of separate capillary elements, microscale chambers, and the like, all of which are connected together to define an integrated body structure. Alternatively and in preferred aspects, the integrated body structure is fabricated from two or more substrate layers which are mated together to define a body structure having the channel and chamber networks of the devices within. In particular, a desired channel network is laid out upon a typically planar surface of at least one of the two substrate layers as a series of grooves or indentations in that surface. A second substrate layer is overlaid and bonded to the first substrate layer, covering and sealing the grooves, to define the channels within the interior of the device. In order to provide fluid and/or control access to the channels of the device, a series of reservoirs or reservoirs is typically provided in at least one of the substrate layers, which reservoirs or reservoirs are in fluid communication with the various channels of the device.
A variety of different substrate materials may be used to fabricate the devices of the invention, including silica-based substrates, i.e., glass, quartz, fused silica, silicon and the like, polymeric substrates, i.e., acrylics (e.g., polymethylmethacrylate) polycarbonate, polypropylene, polystyrene, and the like. Examples of preferred polymeric substrates are described in commonly owned published international patent application no. WO 98/46438 which is incorporated herein by reference for all purposes. Silica-based substrates are generally amenable to microfabrication techniques that are well-known in the art including, e.g., photolithographic techniques, wet chemical etching, reactive ion etching (RJE) and the like. Fabrication of polymeric substrates is generally carried out using known polymer fabrication methods, e.g., injection molding, embossing, or the like. In particular, master molds or stamps are optionally created from solid substrates, such as glass, silicon, nickel electro forms, and the like, using well-known micro fabrication techniques. These techniques include photolithography followed by wet chemical etching, LIGA methods, laser ablation, thin film deposition technologies, chemical vapor deposition, and the like. These masters are then used to injection mold, cast or emboss the channel structures in the planar surface of the first substrate surface. In particularly preferred aspects, the channel or chamber structures are embossed in the planar surface of the first substrate. Methods of fabricating and bonding polymeric substrates are described in commonly owned U.S. patent application Ser. No. 09/073,710, filed May 6, 1998, and incorporated herein by reference in its entirety for all purposes.
Further preferred aspects of the microfluidic devices of the present invention are more fully described in co-pending U.S. patent application Ser. No. 09/238,467, as filed on Jan. 28, 1999 (commonly assigned with the present application), the full disclosure of which is incorporated herein by reference. These preferred aspects include, for example, a reaction zone disposed within the overall body structure of the device, a reagent or other component of an “biochemical system” (generally referring to a chemical interaction that involves molecules of the type generally found within living organisms), sensing systems for detecting and/or quantifying the results of a particular reaction (often by sensing an optical or other detectable signal of the reaction), and the like.
Referring once again to
Microfluidic channels 32 are in fluid communication with each other at channel intersections 34 a, 34 b, . . . (generally referred to as intersections 34). To simplify analysis of microfluidic network 30, channels 32 may be analyzed as channel segments extending between nodes defined at reservoirs 18 and/or channel intersections 34.
To provide enhanced control over movement of fluids within microfluidic network 30 by reducing the effects of secondary hydrostatic forces (such as capillary forces within reservoirs 18), the resistance of channels 32 to flow through the microfluidic network may be enhanced. These enhanced channel resistances may be provided by having a channel length greater than the normal separation between the nodes defining the channel segment, such as by having serpentine areas 36 along the channel segments. Alternatively, a cross-sectional dimension of the channel may be decreased along at least a portion of the channel, or flow may be blocked by a flow restrictor such as a local orifice, a coating or material disposed in the channel, or the like. In general, to take advantage of the full range of flow control provided by the pressure modulators, microfluidic device 12 should be optimized for hydrodynamic flow. Flow control is generally enhanced by providing sufficient flow resistance between each reservoir 18 and the adjacent nodes so as to allow a sufficient variation in flow rate to be achieved within the various channel segments given the dynamic operating pressure range of the pressure modulators.
Pressure manifold 28 can be seen more clearly in
Manifold 28 may comprise a polymer, a metal such as 6061-T6 aluminum, or a wide variety of alternative materials. Lumens 42 may have a dimension in a range from about 2 mm to about 3 mm. Fittings 44 optionally comprise standard ¼-28 fittings. Sealing body 46 will often comprise an elastomer such as a natural or synthetic rubber.
The pressure transmission system (including manifold 28) will preferably maintain a seal when transmitting pressures greater than atmospheric pressure (positive gauge pressures) and less than atmospheric pressure (negative gauge pressures or vacuum). The pressure transmission system and modulator bank 14 will generally be capable of applying pressure differentials which are significantly higher than hydrostatic and capillary pressures exerted by, for example, a buffer or other fluid in reservoirs 18, so as to avoid variability or noise in the pressure differential and resulting flow rates. As capillary pressures within reservoirs 18 are typically less than 1/10 of a psi, often being less than 1/100th of a psi, the system will preferably be capable of varying pressure at reservoirs 18 throughout a range of at least ½ psi, more often having a pressure range of at least 1 psi, and most often having a pressure range of at least +/−1 psig (so as to provide a 2 psi pressure differential.) Many systems will be capable of applying at least about a 5 psi pressure differential, optionally having pressure transmission capabilities so as to apply pressure anywhere throughout a range of at least about +/−5 psig.
A control system for selecting the pressures applied to reservoirs 18 is schematically illustrated in
In general, pressure controller 54 transmits drive signals to an actuator 56, and the actuator moves a piston of displacement pump or syringe 58 in response to the drive signals. Movement of the piston within pump 58 changes a pressure in pressure transmission system 20, and the change in pressure is sensed by pressure sensor 24. Pressure sensor 24 provides a feedback signal to the pressure controller 54, and the pressure controller will optionally make use of the feedback signal so as to tailor the drive signals and accurately position the piston.
To enhance the time response of the pressure control system, pressure controller 54 may include pressure calibration data 60. The calibration data will generally indicate a correlation between drive signals transmitted to actuator 56 and the pressure provided from the pressure modulator. Pressure calibration data 60 will preferably be determined by initially calibrating the pressure change system, ideally before initiation of testing using the microfluidic network.
Generation of calibration data 60 may be effected by transmitting a calibration drive signal to actuator 56 and sensing the pressure response using pressure sensor 24. The change of pressure from this calibration test may be stored in the program as calibration data 60. The calibration signal will typically cause a known displacement of the piston within pump 58. Using this known displacement and the measured change in pressure, the overall pressure system response may be calculated for future drive signals using the ideal gas law, PV=nRT (in which P is pressure, V is the total compressible air volume, n is the number of moles of gas in the volume, R is the gas constant, and T is the temperature). Calibration may be preformed for each modulator/pressure transmission systems/reservoir (so as to accommodate varying reagent quantities within the reservoirs, and the like), or may be preformed on a single reservoir pressurization system as an estimate for calibration for all of the modulators of the system.
Once calibration data 60 has been generated, pressure controller 54 can generate drive signals for actuator 56 quite quickly in response to a desired pressure signal transmitted from network controller 52. It should be noted that these estimate will preferably accommodate the changing overall volume of the compressible gas within the system, so that the calculated change in pressure for a given displacement of the piston within pump 58 at low pressures may be different than the same displacement of the piston at high pressures (i.e., the displacement/pressure correlation plot is not linear, but curves.)
In the exemplary embodiment, actuator 56 comprises a stepper motor coupled to a linear output mechanism. Pump 58 comprises a syringe having a length of about 100 mm, and a diameter of about 20 mm. Overall response time for the system may depend on a variety of parameters, including dead volume, syringe size, and the like. Preferably, the response time will be less than about 1 sec/psi of pressure change, ideally being less than about 500 msecs/psi for a pressure change from zero to 1 psi.
Network controller 52 generally calculates the desired pressure from each pressure modulator in response to a desired flow in one or more of the channels of microfluidic network 30. Given a desired channel flow, network controller 52 derives these pressures using network data 62, with the network data typically being supplied by either a mathematical model of the microfluidic network 64 and/or a tester 66. Network data 62 will generally indicate a correlation between pressure differentials applied to reservoirs 18 and flows within the microfluidic channels.
Network model 64 preferably comprises programming to help translate desired hydrodynamic flow rates into pressures to be applied at reservoirs 18. An exemplary network model 64 generates a hydrodynamic multi-level resistance network correlating to each microfluidic network 30, as can be understood with reference to
Referring now to
Analysis of the multi-level flow resistance network may be performed using techniques often used for analysis of current in electrical circuits, as can be understood with reference to
As described above, flow resistances maybe calculated based upon hydrodynamic chip design alone. It is also possible to measure these resistances using, for example, electrical sensors, pressure drop sensors, or the like. In other words, resistances to hydrodynamic flow of the channels and channel segments may be measured by, for example, measuring electrical resistance between reservoirs 18 while a conductive fluid is disposed within the network. Regardless, once the channel resistances are known, the pressure drop in each channel segment in the network can be obtained by simply multiplying the flow rate of that channel with its associated channel resistance. The pressure of each reservoir 18 can then be calculated by summing up all the pressure drops along the network 30 starting at the top level of the network.
Referring now to the exemplary program for calculating pressures illustrated in
Determination of reservoir pressures so as to provide a desired flow rate will preferably be performed using a pressure calculation program 70, as illustrated in
Electrokinetic/hydrodynamic system 90 also provides the advantages of hydrodynamic transport described above. This hydrodynamic transport is quite reliable, and is independent of charges and electrical surface properties of the channels. Hydrodynamic transport is particularly well-suited for biocompounds which are sensitive to electrical fields.
Electrokinetic/hydrodynamic microfluidic system 90 includes many of the pressurization, microfluidic network and control components described above. In this embodiment, manifold 92 includes fittings 44 opening laterally from the manifold to provide sealed fluid communication from each pressure transmission tube 20 to an associated reservoir 18 of the microfluidic device 12. Additionally, electrodes 94 are coupled to each reservoir 18 via manifold 92. In the exemplary embodiment, the electrodes comprise platinum surfaces which extend down from manifold 92 into electrical contact with fluids disposed within reservoirs 18 when the manifold provides a sealing engagement between fittings 44 and the reservoirs. Coupling of the electrodes with the fittings 44 may be provided by using “T” connectors within the manifold for each well, and inserting a platinum electrode across and through the “T”. The appropriate (upper, in this example) connector branch of the T-connector can be sealed and the electrode affixed in place with a sealing material such as epoxy.
By coupling electrodes 94 to computer 22, and by including within computer 22 an electrokinetic fluid transport controller capable of inducing electro-osmosis and electrophoresis, the system of
One particular advantageous use of the pressure modulated flow control can be understood with reference to
An alternative well-pair dilution plot in
Characterization of an enzyme often involves determination of maximum reaction velocity and a Michaelis constant for each substrate. The enzymatic reaction of Alkaline Phosphatase on dFMUP (as illustrated in
Additional exemplary assay reactions, assay results, and microfluidic networks to provide those results are illustrated in
Referring now to
Accurate control of the flow of fluids within a network of microfluidic channels can be quite challenging within even a relatively simple network of channels. More specifically, in many microfluidic applications, a variety of different fluids (with different characteristics) may be present in a single channel segment. As described above, where the hydro-resistance of each channel segment can be obtained, it may be possible to simulate and calculate the flow of fluids throughout the network for a given pressure configuration. Unfortunately, it can be quite difficult to accurately calculate viscosities (and, hence, resistances and flow rates) when several different buffers are used within a channel, often together with one or more different test fluid samples.
Fortunately, a relatively simple flow sensor can be provided to measure an actual flow within a channel of a microfluidic network. Where the measured flow results from a known driving force (such as a known pressure differential) can be determined, pressures to be applied at the fluid reservoirs so as to affect a desired flow condition may then be calculated.
Referring now to
Detector 154 is downstream from intersection 152, and can be used to detect the arrival time of the signal, for example, as a peak or dip in the intensity of a fluorescent signal from the dye. Thus, the time difference between imposition of the signal at intersection 152 and sensing of the signal flow at detector 154 may be readily measured. Calling this time differential Δt, and knowing the distance along channel 32 between intersection 152 and detector 154, Δd, from the microfluidic network geometry, the flow rate Q can be calculated from the equation:
Where viscosity is to be determined by the system of
A variety of alternative structures may be used to sense flow characteristics so as to apply a proper pressure configuration to generate a desired flow. For example, a signal may be imposed on a flow within a microfluidic channel by photobleaching of a fluorescent dye, rather than imposing a flow perturbation at a intersection. Alternative flow velocimetry approaches such as laser Dopler velocimetry, tracer particle videography, and the like are also possible. Using such techniques, a simple straight channel connecting a fluid supply reservoir and a waste fluid reservoir may suffice, with the fluid supply reservoir containing a fluid comprising a photobleachable fluorescent tracer dye or appropriate tracer particles.
As can be understood with reference to the calculations of flow rate Q above, sensors may also be used to determine alternative flow characteristics within a microfluidic channel, including flow rate, viscosity, the proportionality constant for a segment or network (by use of fluids having known and/or uniform viscosities) and/or other flow characteristics. In fact, in addition to providing a tool to study effective viscosity of two or more mixed fluids (of optionally unequal viscosity) still further measurements are possible. Mixing of DMSO and an acquiesce buffer can yield a non-monotonic viscosity-composition relationship. By applying different levels of pressure differential ΔP and measuring the flow rate Q, viscometer 150 could be used to establish a relationship of the effective viscosity during mixing as a function of mixing length. This information may be pertinent to chip design for tests which involve geometric dilution.
Where temperature dependency of viscosity is of interest, systems such as viscometer 150 can be coupled to a temperature control system comprising an external heater block in contact with the body defining the microfluidic channel network, by using joule heating to selectively control the temperature of fluids within the channel network, or the like. In a still further alternative, a structure similar to viscometer 150 might be used to measure non-Newtonian viscosity. Non-Newtonian fluids have viscosities which are a function of the sheer rate experienced by the fluid. One example of a non-Newtonian fluid is a polymer solution containing high molecular weight molecules. A microfluidic viscometer similar to viscometer 150 of
Real-time flow and viscosity measurements for microfluidic systems based on transient pressure pulse techniques can be further understood with reference to
A steady flow can be directed toward reservoir 18 a by applying initial pressures on wells 18. A short pressure pulse may be applied to well 18 e and/or some or all of the other reservoirs of the microfluidic system. This pressure pulse will propagate substantially instantly to alter flow at some or all of the intersections 34 of network 30. This disturbance of the flow at the node points can change the dilution ratio from one or more of the side branches. After the pressure pulse, steady state flow is resumed.
As can be understood with reference to
Referring now to
Hydrodynamic, electrokinetic, and other fluid transport mechanisms may be used in a variety of ways to provide specialized functions within a microfluidic system. For example, fluid mixtures such as biological fluid samples having particulates and/or cells in suspension within a liquid are often introduced into microfluidic systems. A particularly advantageous system and method for introducing a large number of samples into a microfluidic system is described in U.S. Pat. Nos. 5,779,868 and 5,942,443, the full disclosure of which is incorporated herein by reference. In that system, a vacuum may be used to draw a sequential series of fluid samples from the wells of a multi-well plate into a capillary tube in fluid communication with the microfluidic system.
In the above-described system, it may be desirable to maintain fluids at a substantially stationary location within the microfluidic channel, for example, during the time delay while a sample in a last well of a first multi-well plate is moved away from the capillary tube and before a sample in a first well of a second multi-well plate is in fluid communication with the capillary tube. Maintaining the fluids within the microfluidic channel at a substantially fixed location can avoid introducing significant amounts of air into the microfluidic system, which might interfere with its operation. In general, it may be desirable to maintain fluid mixtures at a given location within a microfluidic network for a wide variety of reasons.
Unfortunately, work in connection with the present invention has found that halting movement of some fluid mixtures within a microfluidic network may have significant disadvantages. Specifically, cell-based assays performed using a fluid mixture including cells suspended in a liquid are susceptible to sticking of the cells to the channel walls if flow is completely halted. Similarly, other fluids may deteriorate if flow within the channel is sufficiently low for a sufficient amount of time.
To avoid deterioration of fluid mixtures, the present invention can provide a small amplitude oscillatory movement of a fluid mixture so as to maintain the fluid mixture within a microfluidic channel. Modulator bank 14 is capable of providing a small amplitude oscillatory pressure such that there is no significant inflow or outflow of materials from the channel. This small amplitude oscillatory pressure will preferably be sufficient to continuously move the fluid mixture (and, for example, the cells within the liquid) continuously back and forth. The oscillation frequency should be high enough such that the instantaneous fluid mixture velocity is sufficiently high to avoid deterioration of the mixture, while amplitude should be small enough such that there is little or no unintended net transportation into or out of the channel from adjacent reservoirs, reservoirs and intersecting channels. Once the desired delay in fluid mixture movement has been provided it will often be desirable to flow an intervening liquid such as a buffer into the channel to help insure that unintended flows and/or mixtures at the channel ends have been flushed.
It should be noted that this small amplitude oscillatory motion may optionally be provided using electrokinetic forces, such as providing an alternating current, particularly if the alternating current is not harmful to cells or other components of the fluid mixture. It may also be beneficial to insure that cells in the channel do not lyse when subjected to the alternating current if electrokinetic forces are to be used to induce the oscillatory motion.
Referring now to
The use of multiple capillary assembly 170 is beneficial for parallel assays using a plurality of test samples, and the like. Referring now to
If, for example, a steady-state flow is induced from capillary 176, a substrate reservoir 180 a, and/or an enzyme reservoir 180 b toward a vacuum reservoir or waste well 180 c along a channel 182, a flow perturbation can be initiated at intersection 186 between the capillary channel and the microfluidic network at the time the capillary is withdrawn out from the well containing the introduced fluid. This flow perturbation may, for example, comprise a change in composition of the flow progressing along channel 182 toward vacuum reservoir 180 c. This change in composition may be sensed at a detection location 184 as, for example, a change in fluorescent intensity. Similar flow perturbations might be induced by applying other pressure transients at intersection 186, for example, when capillary 176 is introduced into the spontaneously injected fluid, or by applying a change in pressure using a pressure modulation pump as described above, again changing the composition of the flow within channel 182. By monitoring the property of the composition at detection point 184, progress of the perturbations may be detected. A time delay between initiation of the perturbation and their respective detections at the detection point, when combined with a known length of channel 182, can be used to determine a speed of the flow within the channel. From this actual, real-time speed, a variety of information regarding the fluid and/or network system may be determined.
Referring now to
Where the channel lengths may be designated, and Δd1, Δd2 a time delay may be measured at detector 184 between initiation of the pressure transient (at t=0) and sensing of a first flow perturbation as a signal 188 a at detector 184. The first signal 188 a may be said to have occurred after a time delay of Δt1, with this time being the time required for flow to propagate from the intersection immediately upstream of detector 184. A similar time delay Δt2 will then be required for the flow to propagate from the second upstream intersection (186 in the simple network of
Referring now to
Referring now to
With voltage between the electrodes off, using only pressure to drive fluids within the network, we can determine velocities along the channels between nodes caused by pressure ν1P, ν2P from:
This electroosmotic velocity may then be used to calculate electroosmotic mobility using the equation:
The multiple capillary assembly and simplified capillary networks of
Referring now to
While the exemplary embodiments have been described in some detail, by way of example and for clarity of example, a variety of modifications, changes, and adaptation will be obvious to those of skill in the art. Hence, the scope of the present invention is limited solely by the appended claims.